The present invention relates in general to detecting a DC offset in an automotive audio system, and, more specifically, to use of a diagnostic signal from a power amplifier to determine output conditions of the power amplifier as they are affected by a DC offset.
Automotive audio systems typically have a power amplifier intended to amplify AC signals, but which will amplify any signal which is presented at the amplifier""s input. Audio speakers, (normally 4 ohms), which are attached to the amplifier can be damaged by certain amounts of DC current. Capacitors are normally used on an input side of the amplifier to remove any DC offset in the amplifier input signal, thereby preventing DC current from flowing to the speakers. In the event these capacitors fail or are inadvertently not present, there can be substantial damage caused to the amplifier and/or the speakers. Currently, most automotive audio systems have protection for shorts but not for DC offsets. Copending application serial No. 199-1348 provides a way for sensing relatively large DC offsets. In some situations, however, it may also be desirable to provide DC offset detection when there is only a small audio signal present or the audio signal is not clipping at a high rate.
The present invention provides automatic detection of a DC offset when AC signals are small or the audio signal is not clipping at a high rate. In one aspect, the method detects a DC offset in an audio signal provided by an audio processing unit to an audio power amplifier, wherein the audio amplifier provides a clip detect signal back to the audio processing unit. The method begins by sampling the clip detect signal to determine if the clip detect is active. A power level of the audio amplifier is detected if the clip detect is active. Next, the power level is compared to a predetermined power threshold. A DC offset is identified if the power level is less than the predetermined power threshold.
If clipping occurs when the audio power is low then the voltage must have been shifted by a DC offset. The present invention utilizes a power detection circuit and a clip detect to monitor the power level and the clipping rate of the audio signal to determine if a DC offset is present.